As a method for producing reduced iron, there is known a process of charging a mixture of iron oxides, such as iron core, and a carbonaceous reducing agent, such as coal, onto a hearth of a rotary furnace or a reducing furnace of the moving hearth type, e.g., of the straight grate type, heating the mixture with radiation heat in the furnace while the mixture is moving in the furnace, and then discharging reduced iron, which is obtained by reduction of the iron oxides with the carbonaceous reducing agent, to the outside of the furnace from the hearth using any suitable discharging means, such as a screw mechanism.
In the case of providing the mixture as agglomerates in the form of, e.g., pellets, however, when the agglomerates are charged onto the hearth, powder generated from the agglomerates with drop impacts, etc. is accumulated on the hearth. The accumulated powder is heated and reduced along with the agglomerates, whereby the accumulated powder becomes powdery reduced iron and the agglomerates become granular reduced iron. The granular reduced iron is discharged out of the furnace by a discharging screw, while the powdery reduced iron is pressed into the hearth surface by the discharging screw. In continuous operation, therefore, a problem has occurred in that, as the amount of the reduced iron powder pressed into the hearth surface increases, the reduced iron powder coheres together under pressing forces repeatedly applied from the discharging screw and forms an iron sheet on the hearth surface. In the reducing furnace of the moving hearth type, since heating and reducing zones are at high temperatures, but a raw-material charging zone and a discharge zone are at relatively low temperatures, the iron sheet formed on the hearth surface tends to crack or warp because of such a temperature difference between the zones. Then, if the discharging screw is caught by the deformed iron sheet, a trouble, such as operation shutdown, has resulted.
A technique for solving the above problem has been previously proposed by the inventors (Japanese Patent 3075721). According to the proposed technique, powder entering a furnace in company with agglomerates and generated from the agglomerates is accumulated on the surface of a hearth to form an iron oxide layer on the hearth, and a discharging device is intermittently or continuously moved toward the furnace ceiling during the operation, while adjusting a gap between the discharging device and the iron oxide layer formed on the moving hearth surface. It is hence possible to suppress powdery reduced iron from being pressed into the hearth surface by the discharging device, and to prevent formation of an iron sheet on the hearth. Further, the accumulated layer of the reduced iron powder is periodically scraped off so that the continuous operation is enabled. The proposed technique is intended to enable the operation to be continued by scraping off an iron sheet formed on the hearth surface and periodically renewing and repairing the hearth surface, but it is not intended to scrape off the hearth itself.
Also, as a method for producing metallic iron, there is known a process of charging a mixture of iron oxides and a reducing material into a reduction melting furnace of the moving hearth type, such as a rotary hearth furnace; heating the mixture with radiation heat in the furnace while the mixture is moving in the furnace; reducing the iron oxides with the reducing material; separating carburized, molten and aggregated slag; cooling reduced iron for solidification to form granular solid metallic iron; and then taking the granular solid metallic iron out of the furnace. In this connection, the inventors have previously proposed in, e.g., Japanese Unexamined Patent Application Publication 2000-144224, a technique for forming a vitreous berth layer made up of iron oxides, carbon and a silica compound on the hearth surface of a rotary hearth furnace, thereby preventing damages of the hearth caused by molten iron. However, since the vitreous layer deteriorates because of slag infiltration (permeation) and erosion when the operation is continued, there still remains a room for improvement to realize stable and continuous operation.
In view of the above-described state of the art, an object of the present invention is to provide a method for producing metallic iron, which can easily remove or repair the surface of a hearth even when metallic iron powder is buried in the hearth surface or even when the hearth surface suffers from slag infiltration and erosion, which can increase an availability factor and maintainability of the hearth, and which is suitably practiced for long-term continuous operation.